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GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L09106, doi:10.1029/2009GL037876, 2009
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MESSENGER and Venus Express observations of the solar wind
interaction with Venus
James A. Slavin,1 Mario H. Acuña,2,3 Brian J. Anderson,4 Stas Barabash,5 Mehdi Benna,2
Scott A. Boardsen,1 Markus Fraenz,6 George Gloeckler,7 Robert E. Gold,4 George C. Ho,4
Haje Korth,4 Stamatios M. Krimigis,4 Ralph L. McNutt Jr.,4 Jim M. Raines,7
Menelaos Sarantos,1 Sean C. Solomon,8 Tielong Zhang,9 and Thomas H. Zurbuchen7
Received 26 February 2009; revised 30 March 2009; accepted 6 April 2009; published 12 May 2009.
[1] At 23:08 UTC on 5 June 2007 the MESSENGER
spacecraft reached its closest approach altitude of 338 km
during its final flyby of Venus en route to its 2011 orbit
insertion at Mercury. The availability of the simultaneous
Venus Express solar wind and interplanetary magnetic field
measurements provides a rare opportunity to examine the
influence of upstream conditions on this planet’s solar wind
interaction. We present MESSENGER observations of new
features of the Venus - solar wind interaction including hot
flow anomalies upstream of the bow shock, a flux rope in the
near-tail and a two-point determination of the timescale for
magnetic flux transport through this induced magnetosphere.
Citation: Slavin, J. A., et al. (2009), MESSENGER and Venus
Express observations of the solar wind interaction with Venus,
Geophys. Res. Lett., 36, L09106, doi:10.1029/2009GL037876.
1. Introduction
[2] The solar wind interaction with Venus has been the
subject of intensive investigation by the Venera 9 and 10,
Pioneer Venus Orbiter, and, most recently, Venus Express
missions [Barabash et al., 2007; Zhang et al., 2008]. Venus
has no measurable intrinsic magnetic field to shield it from
the solar wind, which interacts directly with the planet’s
ionosphere and upper atmosphere. The resulting solar wind
interaction is remarkably complex, with interplanetary conditions and solar extreme ultraviolet irradiance controlling
the production and transport of ionospheric plasma, the
degree of penetration of the interplanetary magnetic field
(IMF) into the ionosphere, and the rate of volatile loss to
1
Heliophysics Science Division, NASA Goddard Space Flight Center,
Greenbelt, Maryland, USA.
2
Solar System Exploration Division, NASA Goddard Space Flight
Center, Greenbelt, Maryland, USA.
3
Deceased 5 March 2009.
4
Johns Hopkins University Applied Physics Laboratory, Laurel,
Maryland, USA.
5
Swedish Institute of Space Physics, Kiruna, Sweden.
6
Max Planck Institute for Solar System Research, Katlenburg-Lindau,
Germany.
7
Department of Atmospheric, Oceanic and Space Sciences, University
of Michigan, Ann Arbor, Michigan, USA.
8
Department of Terrestrial Magnetism, Carnegie Institution of
Washington, Washington, D. C., USA.
9
Space Research Institute, Austrian Academy of Sciences, Graz,
Austria.
Copyright 2009 by the American Geophysical Union.
0094-8276/09/2009GL037876$05.00
solar wind scavenging [Pérez-de-Tejada, 2004; McKennaLawlor et al., 2008].
[3] The close flyby of Venus by the MErcury Surface, Space
ENvironment, GEochemistry, and Ranging (MESSENGER)
spacecraft on 5 – 6 June 2007 offers a special opportunity to
study the Venus plasma environment with Venus Express
acting as a solar wind monitor. These rare two-point measurements provide the first observations of hot flow anomalies in
the Venus bow shock and a large magnetic flux rope
embedded in the cross-tail current sheet, as well as a new
determination of the timescale for magnetic flux transport
through the solar wind interaction region.
2. MESSENGER and Venus Express
Observations
[4] The locations of the MESSENGER and Venus Express (VEX) spacecraft during 5– 6 June 2007 are shown in
Figure 1 using Venus Solar Orbital (VSO) coordinates. In
this system XVSO is directed from the center of the planet
toward the Sun, ZVSO is normal to the Venus orbital plane
and positive toward the north celestial pole, and YVSO is
positive in the direction opposite to orbital motion. Average
Venus bow shock [Slavin et al., 1984] and ion composition
boundary [Martinecz et al., 2008] models that span the
MESSENGER trajectory are included for context. The
MESSENGER spacecraft remained between Z = 2 RV
and +2 RV during the encounter, where RV is the radius of
Venus, while VEX was below the plane of the Venus orbit
returning from apoapsis between Z = 12 RV and 8 RV.
[5] The MESSENGER (black trace) and VEX (red trace)
measurements of magnetic field B are plotted together in
Figure 2. The agreement between the two magnetic field
data sets is within ±1 nT everywhere except, obviously, for
the period when MESSENGER is in the Venus interaction
region itself. The bottom plot of Figure 2a displays the IMF
‘‘clock angle,’’ the angle that the projection of the IMF
vector onto the Y-Z plane makes relative to the +Y direction
measured clockwise as viewed from the +X direction. This
angle is important because it defines the symmetry plane for
magnetic field draping, and it rotated to different orientations throughout the MESSENGER encounter. VEX also
measured the solar wind during the flyby (not shown) and
determined that the solar wind speed, dynamic and static
pressures, and Alfvén and sonic Mach numbers were all
nearly constant and close to their mean values near Venus,
0.72 AU from the Sun, of 420 km/s, 5 nPa and 0.02 nPa,
and 7 and 8, respectively.
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Figure 1. MESSENGER and Venus Express (VEX) trajectories projected into the VSO (a) X-Y and (b) Y-Z planes. Bow
shock and ion composition boundaries from Slavin et al. [1984] and Martinecz et al. [2008] are displayed with 4° of
aberration.
[6] MESSENGER approached from the upstream direction, crossed the dawn-side foreshock, and encountered the
bow shock at 22:57:54 UTC as shown in Figure 2b. After
the dawn magnetosheath crossing the total field increased.
The transition to these strong, low-variance magnetic field
levels marks the crossing of the ‘‘magnetic pile-up boundary’’ (MPB) [Bertucci et al., 2003] and the entry into what
is called the ‘‘magnetic barrier’’ or the ‘‘induced magnetosphere’’ [Zhang et al., 1991]. These current layers are
essentially coincident with the ‘‘ion composition boundary’’
identified in plasma measurements [Martinecz et al., 2008]
and explained in terms of viscous interactions [Pérezde-Tejada, 2004; McKenna-Lawlor et al., 2008]. The clockangle variations in the upstream IMF placed MESSENGER
in the cross-tail current sheet as it moved around the dawn
terminator and entered the near tail. The magnetic tail of
Venus consists of two lobes of strong, oppositely directed,
relatively steady magnetic flux that maps back to the
magnetic barrier [Saunders and Russell, 1986]. The two
lobes are separated by a plasma sheet with a high ratio, b > 1,
of kinetic to magnetic pressure composed of solar wind and
ionospheric plasma and planetary pick-up ions [Slavin et al.,
1989; Barabash et al., 2007]. After multiple encounters
with the cross-tail current layer, MESSENGER remained in
the ‘‘toward’’ (i.e., BX > 0) lobe of the tail before exiting
through a magnetopause-type current layer that forms the
downstream extension of the MPB.
3. Hot Flow Anomalies in the Venus Foreshock
[ 7 ] Ahead of the inbound bow-shock crossing at
22:57:50, a well-developed foreshock was observed with
Figure 2. (a) MESSENGER and Venus Express magnetic field observations are displayed as black and red traces,
respectively, in VSO coordinates from 20:00 UTC on 5 June 2007 to 06:00 UTC on 6 June 2007. (b) A closer view of these
same magnetic field measurements from 22:50 to 00:50 UTC. BS = bow shock; MSH = magnetosheath; MPB = magnetic
pile-up boundary.
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Figure 3. (a) Magnetic field measurements in the Venus foreshock just upstream of the bow shock showing the presence
of two hot flow anomalies. (b) Detailed view of the two HFAs. (c) A schematic diagram, after Schwartz et al. [2000],
showing the geometry and formation process for hot flow anomalies and the direction of the motional electric field
observed by MESSENGER before, Epre, and after, Epost, the passage of the interplanetary (IP) current sheet.
extensive upstream waves, as shown in Figure 3a. Consistent with the wave activity, the angle between the local
magnetic field and the normal to the bow shock determined
from co-planarity, qBN, was generally less than 60°. Embedded
within the foreshock are two groups, centered on 22:54:25
and 22:57:20, of strong, shock-like magnetic field increases
that bound brief, 30– 45 s, intervals of decreases to near
zero field intensity surrounding central peaks that are as
strong as the outer enhancements. Observations at Earth have
shown that these structures, termed ‘‘hot flow anomalies’’
(HFAs), are produced by the interaction of interplanetary
current sheets with back-streaming ions in the foreshock
[Schwartz et al., 2000].
[8] The two hot flow anomalies in the Venus foreshock,
labeled HFA #1 and #2 in Figure 3a, are shown in more
detail in Figure 3b. The central low-magnetic field intensity
regions in the HFAs are believed to be formed by inwarddirected electric fields set up by interplanetary current
sheets passing through the foreshock [Schwartz et al.,
2000]. Necessary conditions for the formation of HFAs
are: (a) the motional electric field must be directed toward
the current sheet on at least one side of the disturbance; and
(b) the current sheet must move along the bow shock at a
sufficiently slow speed so that particles reflected at the bow
shock may propagate upward along the interplanetary
current sheet [Schwartz et al., 2000]. Indeed, examination
of the MESSENGER data shows clearly that these two
HFAs are centered on weak interplanetary current sheets
indicated by the change in the magnetic field before and after
each HFA. Analysis of the magnetic field data around these
cavities, following the approach of Schwartz et al. [2000],
confirms that these requirements are satisfied for the
MESSENGER HFAs. The normals to the interplanetary
current sheets, determined by taking cross products of the
magnetic field before and after the HFA encounters, are found
to be tilted by 60–67° from the Sun-Venus line with an
estimated error of 10°. The motional electric field points
toward the current sheet on either side of the second HFA, but
only on the pre-sector in the first event, explaining why the
field at the edges of the second HFA is much stronger.
Moreover, the ratio of the speed at which the current sheet
‘‘slides’’ along the bow shock to the gyro speed of reflected
particles, again following Schwartz et al. [2000], is found to be
0.5 and 0.4 for the two events, respectively. Values less than
unity imply that the reflected particles have time to move along
the current sheet and to develop an extended cavity. Hence,
there is little doubt that MESSENGER observed Earth-like
HFAs during its passage through the Venus foreshock.
4. Two-Spacecraft Determination of the
Timescale for Draped Magnetic Flux Transport
[9] A full understanding of the solar wind interaction
with unmagnetized bodies such as comets and Venus will
require knowledge of the three-dimensional flow field
[Benna et al., 2009]. Two-point observations of time delay
between the orientation of the magnetic field in the interplanetary medium and the magnetic field convected by the
solar wind into the interaction region is a powerful test of
global numerical models that has been used previously at
comets [Schwingenschuh et al., 1987]. The MESSENGER –
Venus Express encounter geometry and the highly variable
IMF clock angle during this interval are ideal for the
determination of such flow-field time delays. As shown in
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Figure 4. (a) Magnetic field measurements in the Venus near-tail from 23:03 to 23:13 UTC on 5 June 2007 indicating that
MESSENGER entered the plasma sheet and observed multiple current-sheet crossings and a magnetic flux rope at altitudes
of only 300 to 600 km. (b) Schematic depiction of the orientation of the cross-tail plasma sheet and embedded current
sheet at 23:10 UTC adapted from Slavin et al. [1989]. (c) The results of a cross-correlation between the ‘‘clock angle’’ of
the magnetic field projected onto the VSO Y-Z plane with Venus Express in the solar wind and MESSENGER in the
induced magnetosphere of Venus. (d) Hodogram of the magnetic field in central region of the flux rope graphed in the plane
of maximum – minimum variance for 23:09:57 – 23:10:40. (e) Hodogram of the flux rope magnetic field in the plane of
maximum – intermediate variance.
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Figure 1, VEX was located essentially south of Venus and
measured the IMF orientation nearly simultaneously with its
arrival at the nose of the interaction region. Hence, to first
order, there is no need to include time corrections for
the transport between VEX and Venus. The duration of
MESSENGER’s immersion in the draped-field magnetosphere of Venus from the dayside MPB to the exit from
the tail was 28 min. A lagged cross-correlation between
32-s averages of IMF clock angle measured at VEX and
MESSENGER (Figure 4c) shows a strong peak at 8.5 min,
indicating that this is the average amount of time it took
for a change in IMF orientation applied to the front of the
Venus magnetosphere to appear in the draped magnetic
field between X 0 and 3 RV. Presumably the effect of
the IMF direction shift would be seen with smaller delays
closer to the planet, but with the data from just this single
flyby it was not possible to determine reliable time lags
from smaller subsets of the encounter data set. The results
obtained here are nonetheless in good agreement with
similar determinations from Venera 9 and 10 [Yeroshenko,
1979] and Galileo – Pioneer Venus Orbiter [Huddleston et
al., 1996] as well as from magnetohydrodynamic models
[Benna et al., 2009].
5. Near-Tail Magnetic Flux Rope
[10] The magnetic field measurements in Figure 4a display the signature of a well-defined magnetic flux rope
centered at 23:10:18, the first report of one observed in the
Venus plasma sheet. Figures 4d and 4e display hodograms
of the magnetic field variation in the central portion surrounding the peak intensity. The directions of minimum, B1 =
(0.94, 0.05, 0.34), intermediate, B2 = ( 0.33, 0.15, 0.93),
and maximum, B3 = ( 0.10, 0.99, 0.12), variance are
reasonably well defined; the ratios of maximum to intermediate and intermediate to minimum eigenvalue are 37.5 and
3.1, respectively. The non-zero mean B1 = 8.9 nT indicates
that the MESSENGER trajectory through the flux rope was
off-center. The unipolar B2 and bipolar B3 traces are well
behaved and correspond to the axial and transverse components of the flux rope field [Elphic et al., 1980; Vignes et al.,
2004]. The duration of the flux rope, 30 s, is too brief to
be resolved by the MESSENGER Fast Imaging Plasma
Spectrometer (FIPS). However, FIPS measurements in the
surrounding plasma sheet (not shown) indicate a flow speed
of 60 km/s. The estimated diameter of the flux rope is
therefore 1800 km, or 0.3 RV.
[11] Flux ropes with magnetic field signatures similar to
those observed by MESSENGER are frequently observed in
the ionospheres of Venus [Elphic et al., 1980] and Mars
[Vignes et al., 2004]. However, these ionospheric flux ropes
are much smaller and have diameters of order 101 – 102 km.
They are believed to form in velocity shear layers in the
inner magnetosheath or as a result of a Kelvin-Helmholtz
instability at the ionopause [Wolff et al., 1980]. Thus far,
searches for the signatures of reconnection in the cross-tail
current layer at Venus, such as flux ropes, have yielded only
negative results [Saunders and Russell, 1986; Slavin et al.,
1989].
[12] These two possible mechanisms for the formation of
the flux rope seen by MESSENGER differ greatly in the
flux rope orientation relative to the cross-tail current sheet.
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If the flux rope observed by MESSENGER were produced
as a result of interplanetary flux tubes being ‘‘rolled up’’
near the ionopause as they slip about Venus, then the
orientation of the flux rope should be approximately normal
to the cross-tail current layer. Alternatively, if the flux rope
seen by MESSENGER was caused by reconnection in the
cross-tail current sheet, then its central axis should lie in the
plane of the current sheet. The minimum variance analysis
in Figure 4 shows that the orientation of the core magnetic
field at the center of the flux rope is largely in the Z
direction or perpendicular to the cross-tail current sheet.
Hence, the MESSENGER magnetic field measurements
suggest that it was formed not by reconnection, but rather
by the velocity shear or some large-scale instability such as
Kelvin-Helmholtz operating near the interface between the
ionopause and inner magnetosheath [Wolff et al., 1980;
Terada et al., 2004].
6. Summary
[13] The simultaneous Venus Express and MESSENGER
measurements taken on 5 – 6 June 2007 provide a rare
chance to observe the influence of solar wind plasma and
interplanetary magnetic field conditions on the solar wind
interaction with Venus. The MESSENGER and VEX magnetic field measurements clearly show the presence of welldeveloped HFAs in the Venus foreshock, the first time such
structures have been reported at Venus. The importance of
these HFAs at Venus is that they form under a very different
set of solar wind conditions from those seen at other planets
[Øieroset et al., 2001; Masters et al., 2008]. Moreover,
these HFAs may significantly perturb conditions in the solar
wind and influence the downstream interaction with the
ionosphere and exosphere.
[14] The MESSENGER and VEX magnetic field measurements have been used to determine the timescale for a
change in IMF orientation to be transported into the induced
magnetosphere between X 0 and 3 RV. The result,
8.5 min, is similar to those found earlier using Venera 9 and
10 [Yeroshenko, 1979] and Galileo – Pioneer Venus Orbiter
[Huddleston et al., 1996] observations. It also supports the
results of recent fluid modeling of the Venus-solar wind
interaction [Benna et al., 2009].
[15] The MESSENGER magnetic field measurements
also captured a well-defined magnetic flux rope embedded
within the Venus plasma sheet. The orientation of the flux
rope strongly suggests that it is produced by velocity shear
or a large-scale Kelvin-Helmholtz instability [Wolff et al.,
1980; Terada et al., 2004] operating near the interface
between the ionopause and inner magnetosheath.
[16] Acknowledgments. Computational assistance and data visualization support provided by C. Liebrecht are gratefully acknowledged. The
MESSENGER project is supported by the NASA Discovery Program under
contracts NASW-00002 to the Carnegie Institution of Washington and
NAS5-97271 to the Johns Hopkins University Applied Physics Laboratory.
References
Barabash, S., et al. (2007), The loss of ions from Venus through the plasma
wake, Nature, 450, 650 – 653.
Benna, M., et al. (2009), Modeling the response of the induced magnetosphere of Venus to changing IMF direction using MESSENGER and
Venus Express observations, Geophys. Res. Lett., 36, L04109,
doi:10.1029/2008GL036718.
5 of 6
L09106
SLAVIN ET AL.: MESSENGER AND VENUS EXPRESS AT VENUS
Bertucci, C., et al. (2003), Magnetic field draping enhancement at the
Martian magnetic pileup boundary from Mars Global Surveyor observations, Geophys. Res. Lett., 30(2), 1099, doi:10.1029/2002GL015713.
Elphic, R. C., C. T. Russell, J. A. Slavin, and L. H. Brace (1980), Observations of the dayside ionopause and ionosphere of Venus, J. Geophys.
Res., 85, 7679 – 7696.
Huddleston, D. E., C. T. Russell, M. G. Kivelson, and J. G. Luhmann
(1996), Time delays in the solar wind flow past Venus: Galileo-Pioneer
Venus correlations, J. Geophys. Res., 101, 4456 – 4539.
Martinecz, C., et al. (2008), Location of the bow shock and ion composition
boundaries at Venus—Initial determinations from Venus Express
ASPERA-4, Planet. Space Sci., 56, 780 – 784.
Masters, A., C. S. Arridge, M. K. Dougherty, C. Bertucci, L. Billingham,
S. J. Schwartz, C. M. Jackman, Z. Bebesi, A. J. Coates, and M. F.
Thomsen (2008), Cassini encounters with hot flow anomaly-like phenomena at Saturn’s bow shock, Geophys. Res. Lett., 35, L02202,
doi:10.1029/2007GL032371.
McKenna-Lawlor, S. M. P., et al. (2008), Predicting interplanetary shock
arrivals at Earth, Mars, and Venus: A real-time modeling experiment
following the solar flares of 5 – 14 December 2006, J. Geophys. Res.,
113, A06101, doi:10.1029/2007JA012577.
Øieroset, M., D. L. Mitchell, T. D. Phan, R. P. Lin, and M. H. Acuña
(2001), Hot diamagnetic cavities upstream of the Martian bow shock,
Geophys. Res. Lett., 28, 887 – 890.
Pérez-de-Tejada, H. (2004), Plasma channels and electron density profiles
near the midnight plane in the Venus nightside ionosphere, J. Geophys.
Res., 109, A04106, doi:10.1029/2002JA009811.
Saunders, M. A., and C. T. Russell (1986), Average dimensions and magnetic structure of the distant Venus magnetotail, J. Geophys. Res., 91,
5589 – 5604.
Schwartz, S. J., G. Paschmann, N. Sckopke, T. M. Bauer, M. Dunlop, A. N.
Fazakerley, and M. F. Thomsen (2000), Conditions for the formation of
hot flow anomalies at Earth’s bow shock, J. Geophys. Res., 105, 12,639 –
12,650.
Schwingenschuh, K., W. Riedler, Y. Yeroshenko, J. L. Phillips, C. T. Russell,
J. G. Luhmann, and J. A. Fedder (1987), Magnetic field draping in the
comet Halley coma: Comparison of VEGA observations with computer
simulations, Geophys. Res. Lett., 14, 640 – 643.
Slavin, J. A., R. E. Holzer, J. R. Spreiter, and S. S. Stahara (1984), Planetary Mach cones: Theory and observation, J. Geophys. Res., 89, 2708 –
2714.
L09106
Slavin, J. A., D. S. Intriligator, and E. J. Smith (1989), Pioneer Venus
Orbiter magnetic field and plasma observations within the Venus magnetotail, J. Geophys. Res., 94, 2383 – 2398.
Terada, N., H. Shinagawa, and S. Machida (2004), Global hybrid model of
the solar wind interaction with the Venus ionosphere: Ion escape processes, Adv. Space Res., 33, 161 – 166.
Vignes, D., M. H. Acuña, J. E. P. Connerney, D. H. Crider, H. Rème, and
C. Mazelle (2004), Magnetic flux ropes in the Martian atmosphere:
Global characteristics, Space Sci. Rev., 111, 223 – 231.
Wolff, R. S., B. E. Goldstein, and C. M. Yeates (1980), The onset and
development of Kelvin – Helmholtz instability at the Venus ionopause,
J. Geophys. Res., 85, 7697 – 7707.
Yeroshenko, Y. G. (1979), Unipolar induction effects in the magnetic tail of
Venus, Kosm. Issled., 17, 93 – 105.
Zhang, T. L., J. G. Luhmann, and C. T. Russell (1991), The magnetic
barrier at Venus, J. Geophys. Res., 96, 11,145 – 11,153.
Zhang, T. L., et al. (2008), Initial Venus Express magnetic field observations of the Venus bow shock location at solar minimum, Planet. Space
Sci., 56, 785 – 789.
B. J. Anderson, R. E. Gold, G. C. Ho, H. Korth, S. M. Krimigis, and
R. McNutt Jr., Johns Hopkins University Applied Physics Laboratory,
11100 Johns Hopkins Road, Laurel, MD 20723, USA.
S. Barabash, Swedish Institute of Space Physics, Box 812, SE-981 28
Kiruna, Sweden.
M. Benna, Solar System Exploration Division, NASA Goddard Space
Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA.
S. A. Boardsen, M. Sarantos, and J. A. Slavin, Heliophysics Science
Division, NASA Goddard Space Flight Center, Code 670, Greenbelt, MD
20771, USA. (james.a.slavin@nasa.gov)
M. Fraenz, Max Planck Institute for Solar System Research, Max-PlanckStrasse 2, D-37191 Katlenburg-Lindau, Germany.
G. Gloeckler, J. M. Raines, and T. H. Zurbuchen, Department of
Atmospheric, Oceanic and Space Sciences, University of Michigan, 2455
Hayward Street, Ann Arbor, MI 48109, USA.
S. C. Solomon, Department of Terrestrial Magnetism, Carnegie
Institution of Washington, 5241 Broad Branch Road, NW, Washington,
DC 20015, USA.
T. Zhang, Space Research Institute, Austrian Academy of Sciences,
Schmiedlstrasse 6, A-8042 Graz, Austria.
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